A-scan, short for Amplitude scan, is one of the diagnostic tests in the ophthalmology field that uses ultrasound technology. The scan is responsible for measuring the axial length (AL). The axial length is the eyes’ length. This test provides a more detailed view of what’s inside the eye when compared to a routine eye exam.
The A-scan ultrasound projects a single, one-dimensional beam of sound waves through the eye and then measures reflections. A reflection happens at the junction of two different densities. The test uses amplitude scanning, which is based on timed echoes. Its name ‘amplitude scan’ comes from the echoes that are shown on display as tall spikes of high amplitude.
Eye length can also be determined by measuring the space between spikes displayed. The spikes correspond with the surrounding tissue and correspond to the height of the echo. The echo strength, recorded spike height, and how the sound wave returns to the transducer depends on several factors. These factors together give a recording which helps in the delivery of a diagnosis.
When an A-scan is conducted, a corneal scan, which maps your corneal surface and a macular scan, which looks for abnormalities of the macular region, could also be performed.
Also Known As
- A-scan ultrasound
- Ocular ultrasound
- Eye and orbit ultrasound
The A-scan requires a knowledgeable and highly-skilled technician such as:
- An ophthalmologist
- An optometrist
A-scan measures the shape of the eyes’ surface and other masses present. It can characterize internal eye structures, tumor presence, and their composition.
The test is essential because it can help to determine the cause of various eye disorders such as myopia (near-sightedness) and hypermetropia (far-sightedness). However, there are many other less invasive, uncomplicated tests that can be conducted to help in diagnosing these eye disorders.
The most common use of the A-scan is intraocular lens (IOL) power calculation prior to cataract surgery. The A-scan is an essential test taken before cataract surgery. This is because the A-scan calculates the power of the intraocular lens (IOL) that will be implanted during cataract surgery. The human-made IOL replaces the natural lens, which is removed during the surgery.
Each eye is unique, as they differ in size and shape. This means that the power in the IOL implant needed varies depending on the various measurements of that eye. A-scan measurements are considerably accurate. When they are combined with unique computerized calculations, they allow the surgeon to choose the lens implant power that is appropriate for each eye.
An A-scan is most commonly used with a B-scan to differentiate lesions. When the two scans are used together, they help eye specialists get a complete view of a patient’s eye anatomies.
Despite the A-scan being the most preferred test for IOL calculations, it does have some limitations, including:
- The high cost of the procedure.
- Dependent on the patient’s ability to gaze on one area for an extended period.
- The presence of dense cataracts in the patient’s eyes will be difficult to read through.
Preparation & Expectation
The test is usually painless and without complication. No preparation is required of the patient, with most of the test’s accuracy being highly technician-dependent. The procedure takes an average of 15-20 minutes.
Ultrasound biometry may be performed either through:
Contact (applanation) technique:
This is where the test is performed by putting the probe directly on the cornea. It is a widely used technique, but it is slightly uncomfortable for the patient. This is because of the direct probe and eye contact, which increases the chances of corneal erosion. Corneal erosion is where the cornea’s outer layer is scraped or injured.
This is where the test is performed by using a water bath. The probe is separated from the cornea by a saline-filled scleral shell. This technique is considered more superior to the contact technique because it eliminates corneal compression. However, it is also more complicated to perform as the eye technician will have a hard time determining whether or not they are measuring on the axis.
The patient will be asked to lay down/sit comfortably and focus his/her attention on a particular spot. The doctor will then proceed to introduce anesthetic drops in the patients’ eyes to help ease any irritability.
If the doctor uses the contact technique, they will place the probe directly onto the surface of the eye (cornea).
If the doctor opts for the immersion technique, they will spread the lids and place a scleral shell, which is a short, hollow tube on the cornea. The doctor will then fill the scleral shell with a protective gel first followed by a saline solution. S/he will place the tip of the probe into the fluid. The monitor then captures the readings from the probe.
If the immersion technique is used, at the end of the test, the patient naturally blinks and expels the fluid that the doctor had earlier introduced.
The doctor then interprets the information on the monitor and immediately knows the test results.
If the results are normal, the wave spike measures should be within the normal range.
If the results are abnormal, it may indicate:
- Presence of cataracts, tumours or other foreign masses in the eye
- Hypermetropia and myopia
- Other eye disorders, such as retinal detachment and choroidal melanoma
Once the eye specialist has ascertained the eye disease that the patient is suffering from, s/he will prescribe an appropriate treatment plan. In the case of cataracts, s/he will be able to get an accurate IOL power calculation.
Risks & Complications
A-scan is safe without any known complications. The patient’s eyes will be numbed during the procedure. It is important that the patient doesn’t touch or rub his or her eyes before the numbing effects wear out. Touching or rubbing of eyes can lead to scratching the cornea.